Bone screws are utilized in a variety of medical procedures to take advantage of the natural anchoring properties of bone tissue. Such procedures often involve fastening multiple bone fragments of a bone fracture together, or mounting one or more prosthetic elements to the bone in an effort to improve patient mobility. Bone plates are often also utilized in cooperation with a bone screw to provide a more distributed area of compression against the bone being clamped.
The classical bone screw construction comprises an integrally formed fastener having at one end a threaded shaft of a predetermined length to penetrate the bone to a predetermined depth and a fixed radial head formed with a compression flange disposed at the opposite end. In operation, a physician first determines the proper size of bone screw to install from an array of stocked sizes. Once the appropriate size is selected, the screw is implanted into the bone by drilling the threaded end of the bone screw into the bone, for example, across a bone fracture, and utilizing the head as a clamping element.
While the classical bone screw design works well for its intended uses, different orthopedic applications typically require differently sized bone screw shafts or heads in order to adequately anchor or provide proper compression. Consequently, due to the integral nature of the classical bone screw, treatment centers often must stock an assortment of differently sized bone screws in preparation for any type of bone screw application. This tends to create substantial up-front procurement costs to either the supplier or user institution. Moreover, because the classical screw length is fixed, the surgeon must undertake a fairly accurate pre-implant measurement to properly determine the appropriate sized bone screw. Should the measurement prove less than satisfactory, the head may project beyond the expected compression point, requiring removal and replacement of the screw.
In an effort to solve the problems described above, one proposal for a bone screw, disclosed in U.S. Pat. No. 5,628,752 to Asnis et al., implements a two-piece structure including a shaft formed with bone thread at its proximal end and a nut. The shaft is formed with a plurality of axial in-line teeth projecting angularly toward the shaft proximal end. The nut is complementally formed to slide axially along the shaft in the direction of the proximal end and includes an inwardly projecting radial tab or pawl that prevents axial backing-out of the nut by engaging the previously passed tooth. In operation, the screw shaft is drilled into the bone tissue and the nut advanced axially along the shaft until adequate compression between the screw and the nut is obtained. Any excess shaft projecting beyond the nut is typically cut. As a result, the bone screw is adaptable to a variety of bone screw applications due to its variable length.
A second proposal, disclosed in U.S. Pat. No. 5,167,664 to Hodorek, utilizes a similar linear ratcheting feature by including a screw shaft formed with circumferential external teeth. The teeth engage circumferential internal grooves formed within a head to effect unidirectional axial adjustment. Operation of the Hodorek device is similar to that of the Asnis device.
Although the proposals described above work well to minimize the aforementioned problems inherent in a classical type of bone screw, the linear ratcheting feature is incapable of interfacing with conventional surgical power tools. Because of this problem, implanting a bone screw of the linear ratcheting construction often involves more effort and time on the part of the physician. Moreover, non-conventional tooling capable of interfacing with a linear ratcheting bone screw is often substantially more costly than readily available conventional tools.
Thus, the need exists for a bone screw capable of rotationally ratcheting to allow a convenient interface with conventional surgical power tools. The need also exists for a uniform bone screw construction to maximize the reduced costs inherent with wholesale purchasing. The bone screw of the present invention satisfies these needs.